Network densification is one key element to handle the increasing mobile radio traffic demand. Since network densification of homogeneous macro layer has already reached its limit in many cities, heterogeneous networks (HetNet) consisting of small cells (e.g., pico cells) with overlaying macro cells provide a promising network layout to fulfill future requirements.
Pico cells are particularly suited to serve spatially limited areas with high user density which are generally called hotspots. At least as long as frequency bands above 3 GHz are not available for mobile radio communication, pico and macro cells will run on the same carrier frequencies. In that co-channel mode the high performance area of pico cells is in the range of 30-50 m around the pico site. To provide good performance to the mobile radio users this high performance area should match the hotspot and, therefore, the hotspot location has to be known quite precisely. This is also true for other deployment scenarios to serve hotspots, such as vertical sectorization or smart eNodeB antennas.
User localization in mobile radio networks has been a topic for many years for operators and network technology suppliers. But with introducing clear user localization precision requirements for emergency calls in the USA and also since location-based applications have become more popular, improvements of user localization methods have gotten more attention. With the clear trend towards smart phones, the amount of mobile phones equipped with Global Positioning System (GPS) receivers also increases. But even GPS is not reliable everywhere (e.g., urban street canyons, inside of buildings), or the network operators have no access to the GPS data for private security reasons. A relatively old network-based localization method is fingerprinting where the received power from several base stations is used to estimate the user location. Taking account further information, such as timing advance, reference power measurements, etc., the mean localization error is ranged between 30 and 50 m. In LTE Release 9 specific positioning reference signals (PRS) are introduced enabling Time Difference Of Arrival (TDOA) approaches in the downlink. Generally, TDOA-based localization is less precise for non-line-of-sight (NLOS) and at least 3 eNodeB sites have to be involved. Under a realistic interference situation the localization error is not much better than 40 m but assuming that the interfering cells are muted during the measurement period the mean location error could be reduced to approximately 20 m. LTE Rel. 11 enables TDOA in the uplink too. However, it needs some years to reach a sufficient share of user equipments (UEs) supporting LTE Rel. 9 or higher. Thus, for a short term pico site rollout network operators have to rely on fingerprint-based user localization methods with the corresponding localization error in the range of 50 m.
There is a need for simple and precise detection of hot spots, i.e. spatially limited areas with high user density.